Hot running issue on Reliant Scimitar 6a

Probably, but you might need to give a bit more of a clue.

Starts immediately or needs a bit of churning? What's the fuel pump on it, and where?
Is there an inline filter you can see?
Manual or automatic choke?
If manual, what happens if you pull it out a bit?
Have you actually given it a basic service yet?
Idle speed?
What's the temp gauge doing?
Electric fan? Working?
Any fluid loss?

Oh and it has electronic ignition

It's fuel vaporisation .

I had a couple of Montego 1.6s (SU carbs not Webers though) whose running was like your Scimitar's after 30 minutes in traffic then got worse and worse if the traffic stayed bad until the things stopped running and wouldn't restart at all - several litres of cold water over the carb (and rocker cover-mounted fuel pump) got them running again - but only temporarily if the traffic didn't ease .

I also have an MG Maestro 1600 with twin Weber 40DCNFs which runs like a pup in traffic - but it's never done what the Montego above did and thrown the towel in completely in traffic, and it's never needed the "water over the carbs and fuel pump" trick.

Then there is my 309 1600 with single Weber, that gets a bit "hot and bothered" but thankfully the carb has coolant running through it so the engine gets to a certain stage of rough-running (certainly not as bad as the Scimitar, and much better than the MG) - and then stays there .

What fuel are you using in it?

And - more importantly - have you filled up in the last 3 weeks ("summer grade" petrol, which is less volatile than the "winter" stuff, is available from May 15th to Sept 30th every year as a general rule in the UK)?

Does it have a K&N air filter housing or a Ford pan filter? Some owners have had rough running issues with K&N.

I'll second this. I had exactly the same problem on the '72 Corvette I used to own. I tried all sorts to stop the fuel vaporising as several people had said that was the problem. But it turned out to be the coil: It was sited above the inlet manifold and was fine during normal driving, but if I was in stop/start traffic for an extended period - around 30 mintes - it would slowly start to break down because there was not enough airflow to keep it cool. This showed up as the engine starting to run rough and getting steadily worse until it would stall completely. After a period to let it cool it would run fine again. Once I replaced the coil it was perfect.